In recent years, due to the progression of battery technology, the wide use of hybrid automobiles has advanced rapidly. This type of hybrid automobile employs a power supply system in which a motor or the like is driven by a battery, and during deceleration energy is regenerated into the battery. In this type of power supply system, due to the emergence of batteries of new types, reductions in their size and weight, and increase of their specific power, development has proceeded from sealed lead acid batteries to Ni H batteries and then to Li ion batteries. With batteries of all these types, development of the active material of the battery and development of the battery construction for high capacity and high output have been performed in order to enhance the specific energy density, and efforts are being exerted in order to implement increase of the specific power and supply of usable power over a longer time period.
However although efforts are being made in the automobile field to improve fuel efficiency by yet a further level, the tendency is also predicted that, from the present, new fuel efficiency improvement functions will also be added to automobiles that already exist, in order to reduce emissions of carbon dioxide and so on. Due to this, technology is moving in the direction in which it will be considered to be necessary to provide power supplies having lower losses, in other words having lower internal resistance.
If such a low resistance power supply is implemented as a secondary battery, then the problem arises that its maximum output current is low. Due to this, the requirement for a high capacity electro chemical capacitor with which there is no need to limit the output current is increasing, and, as one example thereof, an electric double layer capacitor (EDLC) is generally known. Such an electric double layer capacitor exhibits a characteristic intermediate between that of a capacitor used for smoothing or the like and that of a battery. Moreover, a hybrid capacitor (HC) doped with lithium ions may be suggested as a capacitor of higher specific energy density that exhibits a characteristic intermediate between that of an electric double layer capacitor and that of a battery.
In relation to these capacitors, since the output power density is higher than that of a battery although the energy density is small, accordingly examples are known that are applied to idling stop systems in which instantaneous output is demanded. However generally a hybrid structure with a battery such as a lead acid battery or the like is employed, since the self discharge of such a capacitor is large. A mechanical relay or a semiconductor switching element such as a MOSFET (a metal oxide field effect transistor) or the like is used for the switch that connects the lead acid storage battery and the capacitor.
Now, since the self discharge of a capacitor is large, a large potential difference can easily build up between the lead acid storage battery and the capacitor when, for example, restarting is to be performed after a long term period of storage. When the switch between the lead acid storage battery and the capacitor is turned ON in this state in which a potential difference is present, an excessively great current is drawn from the lead acid storage battery since the internal resistance of the capacitor is small, and this can lead to deterioration of the life of the lead acid storage battery.
As a method for preventing this kind of flow of excessively great current flow, a pre-charging method is known of limiting the charging current by providing a switch with a limiting resistor and another switch in parallel with this switch, and flowing an initial pre-charging current to the capacitor via this limiting resistor (for example, refer to Patent Document #1).
Furthermore, a structure is known in which a pre-charging function is implemented by providing a semiconductor switching element in parallel with the switch instead of the limiting resistor, and by using this semiconductor switching element (for example, refer to Patent Document #2).